The present application relates to optical devices, specifically to tunable filters suitable for lasers and telecommunication devices.
Tunable filter is an important component in optical communications. Tunable filter is often used in a transmitter system to vary the frequency of the laser beam emitted by a modulated laser. Tunable filter is also used to implement dynamic configurations of frequency channels in a wavelength-division multiplexing (WDM) system at a routing point in an optical network. In telecommunication, tunable frequency ranges cover a C-band from 1530 nm to 1570 nm and an L-band from 1570 nm to 1610 nm.
MEMS-based tunable filter can have high resolutions, but their sensitivity to mechanical disturbances makes it impractical to be deployed in the field. Opto-acoustic tunable filter has good stability, but the narrow bandwidth in the optical communications requires long active acoustic-optical region, which makes this type of filters bulky. Moreover, the acoustic waves can generate a lot of heat in operation.
Recently, Etalon-based filter are widely used in telecommunication. Etalon is based on Fabry-Pérot cavity which produces multiple spectral peaks. In conventional Etalon-based filters, a narrow bandwidth is associated with high insertion loss. In particular, liquid crystal Etalon-based filters have large insertion losses because of the high light absorption in liquid crystal materials in the cavities.
U.S. Pat. No. 5,150,236 teaches a tunable liquid crystal Etalon filter that outputs a series of spectral peaks. The spaces between the spectral peaks can be varied by adjusting the length of Etalon cavity. The output light beam has a single transmission peak only in the short wavelength range from 1472 nm to 1528 nm, wherein the tuning stability is very poor due to sharp slope in tuning curve. Moreover, due to its high reflectivity (>95%) in the Etalon cavity, the Etalon filter disclosed in the '236 patent suffers from large insertion loss and is very sensitive to the alignment errors in the two parallel plates that define the Etalon cavity.
There is therefore a need for a tunable filter that is compact, stable, and has low insertion losses, and can produce single spectral peak in the wavelength range required by optical communications.